The invention relates to a method of manufacturing optical fibers. In the method, the inner wall of a tube is etched with a fluorine compound. Then, a core glass is chemically deposited on the etched surface. The core glass is deposited from the gaseous phase while using a nonisothermal plasma for initiating the reaction.
Core glass is to be understood to mean, in this connection, the glass which is deposited on the inside of the tube. This includes such layers deposited from the gaseous phase which may be indicated as matching layers and which as such do not belong optically to the core of the fiber. The core of a fiber is to be understood to mean that part of the optical fiber in which the refractive index increases by a step (stepped index) or gradually (gradient index) with respect to a part of the fiber surrounding the core.
The tube used in the method may consist of glass, including doped or undoped fused silica.
A chemical deposition process utilizing a nonisothermal plasma is to be understood to mean herein a process in which a cold plasma is reciprocated in a tube while oxygen and silicon tetrachloride are passed through the tube. A dopant which increases or decreases the refractive index of the deposited glass may be added to the gas mixture. The gas pressure in the tube is less than 1.5.times.10.sup.4 Pa (150 mbar), and is preferably between 10.sup.2 and 3.times.10.sup.3 Pa (1 and 30 mbar). During the deposition the glass tube is heated to a temperature between 1000.degree. and 1200.degree. C. At this temperature a thermal reaction, which might lead to the formation of glass soot in the gaseous phase, does not occur at the pressures used. Moreover, the incorporation of chlorine in the deposited glass layers is avoided by this heating. During the steps of collapsing the inner-coated tube into a preform and drawing fibers from the preform formed during collapsing, incorporated chlorine may give rise to gas bubbles. The heating during the deposition also prevents the formation of cracks in the deposited layers in the collapsing step.
In the manufacture of optical glass fibers, it is usual to thoroughly clean the inside of the glass tube or doped or undoped fused silica tube serving as a substrate. A known cleaning method comprises rinsing with a hydrofluoric acid solution in which a small thickness of the tube is etched away. After the etching treatment, prolonged rinsing with water and drying is carried out.